Radiation-induced ionic polymerization controlled by the presence of lewis acids or lewis bases

ABSTRACT

This invention relates to a method for controlling the molecular weight of a polymer derived from an ethylenically unsaturated monomer which can be catalyzed to polymerize by a Lewis acid, which comprises mixing said monomer with a Lewis base soluble in said monomer, and then reacting the mixture with an amount of high-energy radiation at least sufficient to initate and propagate polymerization of said monomer, said Lewis base being selected from the class which functions to determine the molecular weight of the polymer produced at a given radiation dosage and temperature, said monomer being selected from an ethylenically unsaturated monomer having electron-releasing groups attached directly or indirectly to the ethylenic linkage of said monomer which impart a Lewis base character to said monomer. The invention is also applicable to controlling the molecular weight of a polymer derived from a monomer which can be catalyzed by a Lewis base. In that case, the molecular weight of the final polymer can be controlled by mixing the monomer with a Lewis acid and then irradiating the mixture at a given temperature and to a given radiation dosage.

United States Patent [72] Inventor Thomas F. Williams Knoxville, Tenn.[21] Appl. No. 687,410 [22] Filed Nov. 9, 1967 [45] Patented Oct. 26,1971 [73] Assignee The United States of America as represented by theUnited States Atomic Energy Commission Continuation-impart ofapplication Ser. No. 324,146, Nov. 15, 1963, now abandoned.

[54] RADIATION-INDUCED IONIC POLYMERIZATION CONTROLLED BY THE PRESENCEOF LEWIS ACIDS OR LEWIS BASES 2 Claims, 3 Drawing Figs.

[52] US. Cl ..204/l59.22, 204/l59.23, 204/159.24, 260/80 P [51] Int. ClB01j l/00, C08d1/00,BOlf11/00 [50] Field otSearch 204/159.22

[56] References Cited UNITED STATES PATENTS 2,903,404 9/1959 Oita et al.204/l59.24 2,924,561 2/1960 Schmerling 204/159.24 2,943,987 7/1960Anderson 204/15924 FOREIGN PATENTS 1,337,235 8/1963 France OTHERREFERENCES Bonin et al., Effect of Ammonia on the Radiation InducedPolymerization of Cyclopentadiene at 78 Jrn. a/T he American ChemicalSociety, Vol. 84, p. 4355 (11-20-68).

Primary Examiner- M urray Tillman Assistant Examiner-Richard B. TurerAttorney- Roland A. Anderson ABSTRACT: This invention relates to amethod for controlling the molecular weight of a polymer derived from anethylenically unsaturated monomer which can be catalyzed to polymerizeby a Lewis acid, which comprises mixing said monomer with a Lewis basesoluble in said monomer, and then reacting the mixture with an amount ofhigh-energy radiation at least sufficient to initate and propagatepolymerization of said monomer, said Lewis base being selected from theclass which functions to determine the molecular weight ofthe polymerproduced at a given radiation dosage and temperature, said monomer beingselected from an ethylenically unsaturated monomer havingelectron-releasing groups attached directly or indirectly to theethylenic linkage of said monomer which impart a Lewis base character tosaid monomer.

The invention is also applicable to controlling the molecular weight ofa polymer derived from a monomer which can be catalyzed by a Lewis base.In that case, the molecular weight of the final polymer can becontrolled by mixing the monomer with a Lewis acid and then irradiatingthe mixture at a given temperature and to a given radiation dosage.

PATENTEUnm 26 I911 SHEET 2 BF 2 We) 'ANOO INVENTOR. Thomas E WilliamsATTORNEY.

BACKGROUND OF THE INVENTION The present invention relates to an improvedradiation-induced cationic polymerization process. More specifically, itrelates to an improved way of utilizing radiation to control the extent(i.e., the molecular weight) or polymerization of a class of monomerswhich are known to be initiated or catalyzed by Lewis acids includingFriedel Crafts catalysts.

It is known that a selected class of polymerizable monomers can bepolymerized by Lewis acid chemical catalysts. The Friedel Crafts type ofLewis acid catalyst is especially useful for this purpose. Among thecatalysts which are know to be useful for this purpose are included BFAlCl AlI UCI TiCl Z&CL., and other metal halides; BF etherate; hydrogenacids such as H SQ HF, HCl and I-IBr. The polymerization process whichproceeds by virtue of the Lewis acidcatalyzed reaction of selectedpolymerizable monomers is known as cationic polymerization.

In general, cationic polymerizations are characterized by an extremelyrapid rate of polymerization at extremely low (i.e., sub zero)temperature conditions with the efficiency (meaning conversion per unitweight of catalyst) of polymerization decreasing with increasingtemperature owing to competitive side reactions. Cationicpolymerizations are to be contrasted with free radical polymerizationreactions which are known to be inhibited by molecular oxygen or bysmall concentrations of phenols or aromatic amines or other free radicalscavengers. Cationic polymerizations are not affected in the same mannerby these reagents. Another distinguishing feature of chemicallycatalyzed cationic polymerizations is that they often requireheterogeneous catalyst systems, that is to say where the catalyst is oflimited solubility in the polymerizable monomer so that the extent ofpolymerization, i.e., the molecular weight of the polymer formed israther independent of the initiator or catalyst concentration. Adisadvantageous characteristic of cationic polymerization using chemicalcatalysts to induce polymerization is that they are extremely sensitive,difficult to control, and therefore difficult to conduct in areproducible manner.

SUMMARY OF THE INVENTION It is accordingly an object of this inventionto provide an increased measure of control of the polymerization ofmonomers which are known to be initiated and/or catalyzed lyconventional Lewis acids. Another object of this invention is to providean improved process for conducting ionic (including cationic andanionic) polymerizations. Another object is to provide aradiation-induced cationic polymerization process. As used herein theterm polymerization includes homopolymerization as well ascopolymerization.

As a radiation-induced cationic polymerization process the objects ofthis invention are effected by irradiating a mixture of a Lewis base anda monomer, known to be polymerizable by a Lewis acid catalyst, with atleast sufficient radiant energy to initiate and propagate polymerizationof said monomer. The irradiation may be accomplished by X- or gammarays, beta rays, high-energy electrons, accelerated ions, neutrons andthe like. Radiation from the relatively long-lived radioactive fissionproducts frequently provides a convenient beta and gamma source,provided the source in all cases is properly calibrated.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings,

FIG. 1 shows the effect of varying amounts of ammonia on the conversionof cyclopentadiene polymer by irradiating mixtures of said monomer andammonia;

FIG. 2 compares the effect of ammonia with other Lewis bases (i.e.,methyl amines) on the conversion of monomer to polymer; and

FIG. 3 depicts the effect of I-IBr on the conversion of nitroethylene byirradiating mixtures of nitroethylene with varying amounts of I-IBr.

RADIATION-INDUCED CATIONIC POLYMERIZATION The class of suitable monomersfor carrying out this invention, (in addition to being initiated topolymerize by Lewis acids, particularly of the Friedel Crafts type), arecharacterized in that they are ethylenically unsaturated structuresbearing electron-releasing groups attached directly or indirectly to thecarbon atoms of the ethylenic linkage. By a Lewis acid or base, I meanto refer to the acid-base system as propounded by G. N. Lewis and asexplained in Luder, W. F. and Zutfanti, The Electronic Theory of Acidsand Bases," l- 106, John Wiley & Sons, Inc., 1946. With reference tothis invention, a Lewis base is a reagent which possesses unsharedelectron pairs in its molecule and hence will be interacted withelectron deficient sites of a contacting molecule in this case thepositive ions of at least some of the irradiated monomer moleculeshaving electron deficient sites. Among the Lewis bases which areoperable for the purpose of controlling a radiation-induced cationicpolymerization are: ammonia, lower alkyl amines such as methyl amine,dimethyl amine, trimethyl amine; piperazine, piperidine, N-alkylpiperidines, pyridines, pyrroles, straight chain and branched chainalkyl amines containing up to 18 carbon atoms; hexamethylene diamine,ethylene, diamine, N, N dimethyl formamide, hydroxylamine, hydrazine,guanidine; lower alkyl liquid alcohols such as ethyl alcohol, n-butanol;lower alkyl ethers; tetrohydrofurans and phosphines.

A radiation-induced cationic polymerization is similar to a chemicallycatalyzed cationic polymerization in that it is applied to monomershaving a Lewis base character, that is monomers which have electron-richsites. In order to induce formation of electron-rich sites on themonomer, or stated another way, in order to induce a structure tofunction as a Lewis base, electron-releasing groups should be attacheddirectly or linked indirectly to the carbon atoms of the ethylenic bond.Among some of the more common electron releasing groups which can beused to promote a radiation-induced cationic reaction are:

alpha or beta naphthyl, and anthracyl. In general, the orthoparadirecting substituents in benzene rings, when attached directly orindirectly to the carbon atoms of the bond, will impart a Lewis basecharacter to the ethylenic structure.

Typical monomers known to be polymerized by way of CI-l C(OR) cationicpolymerization process in that such monomers are initiated to polymerizeby Lewis acids or Friedel Crafts reagents and which therefore are usefulin the method of this invention are isobutylene, alkyl vinyl ethers CHCHOR where R is a straight or branched chain having up to 18 carbonatoms, such as isopropyl vinyl ether, and isobutyl vinyl ether andoctadecyl vinyl ether, styrene, butyl vinyl ether, p-methoxy styrene,p-dimethylamino styrene, alpha-substituted styrenes, such as alphamethyl styrene, beta pinene, l-3 butadiene, n-vinyl carbazole,vinylidene ethers Cl-l=C(OR) where R CH C H n-C H iso C l-1,, vinylnaphthalenes, vinyl pyridines, 2-vinyl dibenzylfuran, cyclopentadiene,terpenes, myrcene and unsaturated hydrocarbons derived from crackingpetroleum wherein each of said monomers have electron-releasingsubstituents such as to render the electronegative or basic in the Lewisbase sense. Polymerizable monomers having a l, 2 oralpha-beta-substituted structure will also proceed by aradiation-induced cationic polymerization. Among monomers of this kindare those with an indene, acenaphthylene, coumarone structure; 1, ldiphenylethylene, 1, 2 dimethoxyethylene and anethole.

By means of my discovery that ionizing radiation can effectpolymerization of the hereinbefore designated class of monomers, it isnow possible to control the molecular weight of the final polymer. Thus,by my invention, one may produce a host of products from a singlemonomer having a discrete range of molecular weights in a mannerheretofore not thought possible by conducting polymerization of theselected class of monomers through the use of Lewis acid chemicalcatalysis.

In order to practice this invention, a monomer of the defined class ismixed with a Lewis base and the resultant mixture exposed to at leastsufi'icient radiation, as measured in electron volts per gram (or rads)of monomer, to initiate and propagate polymerization. The determinationof the amount of absorbed energy necessary to induce a definite amountof polymerization of a given monomer of the defined class is a matter ofroutine skill within the art and varies with the monomer underconsideration. Further amounts of radiation will in the presence of, orabsence of, the Lewis base, merely result in an increased degree ofconversion. That is, it will result merely in an increase in the amountof monomer converted to a given polymer. However, in the case where theLewis base is used in a mixture with the monomer, the molecular weightof the resultant polymer will be found to be a function of theconcentration of the Lewis base in the preirradiated mixture. Regardlessof the degree of conversion of the monomer, the degree of polymerization(i.e., the molecular weight of the resultant polymer) will remainessentially the same. Thus, any additional energy absorption by themonomer will only result in polymerization of additional monomeric unitsto greater yields of a polymer within a rather discrete range ofmolecular weight.

In general, the kinetics of cationic polymerization initiated by Lewisacid catalysis are most efficient at low, i.e., subzero, temperatures.in some cases, the full extent of polymerization to yield a highmolecular weight polymer takes place virtually instantaneously..Productsof lower molecular weight are possible to achieve by conducting thepolymerization at higher temperatures, but the reproducibility andcontrol of the reac' tion is then difficult to maintain. By contrast,the method of this invention permits operation of ionic (cationic aswell as anionic) polymerization at temperatures where the polymerizationkinetics are rapid, but at conditions under which the degree or extentof polymerization may be controlled within discrete and operable limits.

It should be noted that the discrete control achieved by the presence ofa Lewis base in admixture with an irradiated monomer of the definedclass has no parallel in the Lewis acid chemically catalyzed cationicpolymerization. Thus, addition of a Lewis base to a Lewis acid-catalyzedcationic polymerization would simply neutralize the Lewis acid and thushinder, if not totally impede, the polymerization reaction.

The mechanism connected with the radiation-induced ionic polymerizationof this invention is not thoroughly understood. Apparently, the energyabsorbed from the impinging radiation creates cationic species out of atleast a portion of the irradiated monomeric units. These cationicspecies are thought to initiate and propagate the resultantpolymerization in a manner similar to Lewis acid-catalyzed cationicpolymerization. That these initiating and propagating species arecationic in character is shown by the fact that the extent ofpolymerization depends on the concentration of the added Lewis basereagent, thus making it reasonable to assume that the increased measureof control made possible by this invention is a neutralization reaction.Further evidence of the ionic character of these radiation-inducedreactions is provided by the fact that they are essentially unaffectedby the presence of free radical-inhibiting reagents.

The control which this invention permits is both unique and surprisingfor it is generally known that acid-base neutralizations are extremelyrapid. On this basis, one would expect that polymerization would bepretermitted by the Lewis acid-base neutralization. Apparently, however,the time required for polymerization of the monomer molecule is shorterthan the kinetics involved in the neutralization of the positivelycharged sites on the initiating monomeric units, thus permittingpolymerization in the face of a competing neutralization reaction.

Having described the essential elements and scope of my invention ingeneral terms, the following example, in its several aspects, willfurther define the scope of this invention and illustrate its uniqueoperation as a radiation-induced preparative technique applied to atypical monomer.

EXAMPLE l Cyclopentadiene monomer is known to be easily polymerized to ahigh molecular weight polymeric product by a Lewis acid-catalyzedreaction. See for example, Wilson and Wells, Chemical Reviews, 34, 1,1944. l have shown that a cyclopentadiene monomer can be polymerized byionizing radiation, particularly gamma radiation, such as from a cobalt-60 source. The radiation-induced polymerization product is similar tothe product produced by a Lewis acid catalyst. As is the case withpolymerization by the Lewis acid catalyst route, radiation-inducedcatalysis of monomers of the selected class, including cyclopentadiene,could not be conveniently controlled (at least prior to this invention)to produce a polymer having a discrete range of molecular weight.

A sample of dicyclopentadiene dimer was thermally cracked to the monomerby boiling in a still-pot under a stream of nitrogen. The monomer wasthen passed down a column of activated silica gel into a receivingflask. The monomer was then degassed at a temperature of 78 C. With thepurified cyclopentadiene monomer now held at 78 C. a major portion wasdistilled into another receiving flask maintained at l96 C. the residuebeing rejected. The purified cyclopentadiene was then distilled intoradiation vessels as needed.

The following experiments were conducted with the thus purifiedcyclopentadiene. All irradiations of cyclopentadiene were carried out at-78 C. at calibrated positions relative to a cobalt-60 gamma source at adose rate of about 3X10 rads/min. All dose rate values are based on theuse of the Fricke dosimeter according to the absolute calibration ofHochanadel and Ghonnley as described by them in The Journal of ChemicalPhysics," l953, 21, 880.

l. A portion of the purified cyclopentadiene was transferred into anirradiation vessel and exposed to the cobalt-60 gamma source up tovarying total dosages. Irradiation beyond a total dose of 6.0 l0ev./grams, corresponding to a 2 percent conversion to polymer (in termsof percent monomer converted), resulted in the formation of a polymericgel. it was found that cyclopentadiene monomer polymerized with highyield as evidenced by a G(C H value of about 25,000where G( C Hdesignates the total number of monomer molecules of cyclopentadienewhich disappear per 100 electron volts of absorbed energy. It was foundthat the yield of cyclopentadiene monomer was linear with irradiationdose up to 20 percent conversion, and at doses exceeding 5.0 X ev./g.(i.e., about one megarad), the pure monomer is converted to a firmrubber, impervious to organic reagents which serve as a solvent for thestarting monomer. A total dosage of 1 l0 ev./gram resulted in 3.4percent conversion to polymer; a total dosage of 5X10 ev./gram resultedin 15.2 percent conversion to polymer and a total dosage of 10X 10ev./gram resulted in 25.5 percent conversion.

2. A 5X 10 M solution of the stable free radical 2,2dipheny1-l picrylhydrazyl (DPPH) in cyclopentadiene was irradiated to varying totaldosages. An average G(monomer) value of 8,400 was calculated fromseveral runs.

3. cyclopentadiene samples containing added oxygen (1.7Xl0 moles 0 perliter of cyclopentadiene) were irradiated as in (2) and yield an averageG(monomer) value of about 5,300. These yields obtained in the presenceof DPPH and oxygen are less than those obtained for irradiation of thepure additive-free monomer by a factor of about 3-4 indicating thatradiation-induced polymerization of cyclopentadiene is very littleinhibited by these conventional free radical scavengers.

4. In contrast to (2) and (3) I have irradiated numerous samples ofcyclopentadiene containing varying amounts of Lewis bases such asammonia, methyl amine, dimethyl amine, and trimethyl amine, and, atcomparable concentrations of Lewis base, the G(monomer) value is lowerby several orders of magnitude to values varying from 200-500. Theseresults indicate the strong effect of the Lewis base additive and pointsup that the observed polymerization involves an ionic mechanism ratherthan a free radical mechanism.

It should be pointed out that the selection of monomer to be used in thepresent invention depends not only on its structure in accordance withguidelines heretofore disclosed, and on the fact that such monomer isselected from those monomers which are polymerized by the conventionalchemical catalysts mentioned, but also depends on the temperature ofpolymerization. Thus, the radiation-induced polymerization of somemonomers will proceed by way of a free radical mechanism at onetemperature as evidenced by an extremely low G(monomer) value in thepresence of conventional free radical scavengers; whereas at another(usually lower) temperature, the same monomer will polymerize to yieldan extremely higher (at least by a single order of magnitude) G(monomer) value, indicating that it polymerized by an ionic process.Therefore, in selecting the monomer for the process of this invention,the temperature of polymerization should be considered an importantfactor. Moreover, it shows that despite the fact that a monomer willpolymerize by way of a free radical path under one set of conditions, itdoes not rule out its use in a radiation-induced ionic polymerization asherein described under different conditions.

5. Measurements over the dose rate range 2.70Xl0' ev./gram/ min. to2.83X10" ev./gram/min. indicate that the radiation-induced cationicpolymerization is proportional to (dose rate 6. Radiation-inducedcationic polymerization may be conducted with solutions of the monomerin organic solvents such as hexane. Solutions of cyclopentadiene inn-hexane are easily polymerized by radiation to give G(monomer) valuesof 9,840 for a 25 mole percent cyclopentadiene solution and 4,920 for a75 mole percent solution; these solutions being comparable to thatobtained for the radiation-induced polymerization of the pure monomer atthe same dosage of radiation. In general, radiation-induced cationicpolymerization may be carried outwith solutions of the monomer where thesolvent is neutral; that is, where the solvent is not categorized eitheras a Lewis base or acid such as saturated or chlorinated hydrocarbons.

7. Samples of the purified monomer were transferred to severalirradiation vessels, each containing a different concentration ofammonia admixed with the monomer. These samples were irradiated in thesame manner to the same total dosage 1X10 ev./g. The effect of ammoniaon the degree of conversion is illustrated by the data in table 1 below.

Although these G(monomer) values obtained in the presence of ammonia areless than in the case of the pure monomer, nevertheless these values arecomparable to (and in some instances greater) than those obtained forbutadiene, a monomer known to polymerize to high yields by radiation.

The effect of ammonia concentration on the extent of monomer conversionmay be seen from the curves of FIG. I. In FIG. I, the abscissa parameteris radiation dosage plotted in terms of the total amount of radiationenergy absorbed by the irradiated monomer; the ordinate is plotted interms of percent volume contraction of the liquid monomer wherein a 1percent volume contraction of the liquid monomer corresponds to about a5.7 volume percent of polymer produced. It will be seen that for lowinitial ammonia concentrations, the curvature increases sharply withprogressive irradiation dose indicating that the ammonia may beincorporated in the polymer molecule. By contrast, the contraction curvefor pure cyclopentadiene remains essentially linear with dose over theshort interval to the gel point. The volume contraction is linearlyproportional to the polymer yield (which in this case is 5.7 percentpolymer per 1 percent volume contraction of monomer liquid) asdetennined by direct weighing of the polymer remaining after removal ofthe monomer by distillationthis proportionality factor applied to runscarried out in the presence and absence of ammonia, the Lewis baseadditive used in this experiment.

8. The molecular weight of the polymer isolated from the ammonia runsdepends upon the irradiation dosage and the amount of ammonia added. Athigh ammonia concentrations and low conversions (1 percent or less) thepolymer is a waxy resin, soluble in organic solvents. On the other hand,a cyclopentadiene solution containing an extremely low (i.e. 2X10 molefraction) of ammonia yields a gellike polymer after the sample isirradiated to a dose of 2.0X10- electron volts/gram (3.2 megarads), adosage corresponding to 12.2 percent conversion. The potency of ammoniain controlling the molecular weight of the final polymer is shown by thedata in table 11 below.

TABLE II Ammonia Concentration M, Molecular Weight 2.5X10"M 17,8001.79X10"M 30,900 1.29X10M 40.500 7.5X104 47,000 2.5X104 107,000 2.3X104151,000 no ammonia 580,000

M, was determined from measurements of intrinsic viscosity.

It will be seen that there is a definite correlation between themolecular weight of the polymer and the Concentration of the addedammonia base at a given total radiation dose. The molecular efficiencyof other Lewis base additives used to control the molecular weight ofirradiated polymer at a given radiation dosage will vary according tobasicity of the monomer being irradiated relative to the basicity of theLewis base additive.

9. The preceding experiment (8) has shown the effect of varyingconcentrations of ammonia on the molecular weight of polymer products atthe same applied radiation dosage. However, ammonia, as previouslydescribed, is not the only example of a Lewis base which may be used tocontrol the molecular weight of the irradiated polymer. The organicamines can be utilized to produce parallel results.

10. FIG. 2 depicts the effect of comparable concentrations of ammoniaand a primary, secondary, and tertiary amine of polymer yield, i.e., onthe extent of conversion to polymer, where each point on the variouscurves represents a gravimetric determination on a separate irradiatedsample. It will be seen that comparable amounts of polymer are producedby the addition of ammonia, methylamine, dimethylamine or trimethylamineat the same concentration.

It will be seen that there has been described a radiation-inducedcationic polymerization process in which the final molecular weight canbe controlled within fairly discrete limits simply by including a Lewisbase with the selected monomer to be irradiated.

RADlATlON-INDUCED ANlONIC POLYMERIZATION The process of this inventionis also applicable to control molecular weights of polymers byradiation-induced anionic polymerization of selected monomers. in suchcases the choice of monomer should be such that at least a portion ofthe irradiated monomer molecules will be converted to an anionic speciesby radiation to initiate and propagate the polymerization of themonomer, the existence of said anionic species being evident by the factthat the addition of a Lewis acid will control the molecular weight ofthe polymer as a function of the Lewis acid additive.

A radiation-induced anionic polymerization is similar to a chemicallycatalyzed anionic polymerization in that it is applied to monomershaving a Lewis acid character-that is, monomer having electron deficientsites at the ethylenic linkage. Among some of the knownelectron-attracting groups which can be used to provide aradiation-induced anionic reaction in order to induce structure tofunction as a Lewis acid are -CN, CONl-h, N COOR where R is an alkylgroup, said groups being attached directly or indirectly to the carbonatoms of the ethylenic linkage. Typical monomers suitable for conductinga radiation-induced anionic polymerization are acrylonitrile,methacrylonitrile, vinylidene cyanide, acrylamide, 1,3-butadiene andsubstituted dienes such as isoprene, 1,2 and 1,4 dihydronaphthalenes,styrenel -chlorol nitrothylene, nitroethylene, Z-nitropropylene, methylmethacrylate, methyl acrylate, ethyl acrylate, and n-butyl acrylate.Among the Lewis acids which are operable for the purpose of controllingmolecular weight of polymer in a radiation-induced anionicpolymerization are hydrogen acids including hydrogen bromide, hydrogenchloride, carbon dioxide, alcohols R-OH, glycols, amides, protonicsolvents, alkyl or aryl derivatives of boron such as triethylboron ortriphenylboron which possess electron deficient centers.

EXAMPLE 1! This example is designed to illustrate a representative caseof radiation-induced anionic polymerization. The example is more fullydescribed in Transactions of the Faraday Society, 63, 376 (1967). Inthis example, nitroethylene represents a monomer known to be catalyzedby a Lewis base to polymerize in a nonirradiated induced polymerization.The nitro group creates a cation deficiency at the ethylenic linkage.

Nitroethylene was distilled under reduced pressures. A middle portionboiling between 38-39 C. at mm. Hg was collected and extensively dried.After drying over Drierite, the monomer transferred on to barium oxideat 350 C. in vacuum for several hours and then stored for 24 hoursduring which time slight polymerization occurred. After this storage,the remaining monomer was distilled in vacuum and condensed into severaldilatometer bulbs. Varying amounts of hydrogen bromide (from 0-l0"M HBr)were then added to the bulbs after which they were irradiated at roomtemperature 10 C.) by gamma rays from a 1,000 curie cobalt-60irradiation source. The polymerization was followed by a dilatometricmethod.

After irradiation, the polymer was precipitated by pouring into anacidified water-methanol mixture. The polymer was isolated by filteringand then washed successively with water, ethyl alcohol, and ether beforedrying to constant weight in vacuum at room temperature. The molecularweight of the polymer from each of the bulbs was then measured byintrinsic viscosity measurements.

The effects of l-lBr on the degree of conversion of nitroethylene topolymer is shown in H6. 3. Whereas the conversion is virtuallyasymptotic for the monomer alone, the addition of HBr showsan inhibitionon the amount of conversion followed by a marked increase in rate ofconversion on continued radiation.

The potency of HBr in controlling molecular weight of nitroethylenepolymer is shown in table lll below.

TABLE lll Hydrogen Bromide Concentration Molecular moles/liter Weightwhere R may represent CH or C l-l alpha or beta naphthyl, and anthracyl,attached to the ethylenic linkage of said monomer and b. A Lewis base,soluble in said monomer, at a molecular weight determiningconcentration, said Lewis base being selected from the group consistingof ammonia, lower alkyl amines, piperazine, piperidine, N-alkylpiperidines, pyridines, pyrroles, straight chain and branched chainalkyl amines containing up to 18 carbon atoms; hexamethylene diamine,ethylene, diamine, N, N dimethyl formamide, hydroxylamine, hydrazine,guanidine; lower alkyl liquid alcohols, lower alkyl ethers;tetrohydrofurans and phosphines, irradiating each mixture withhigh-energy radiation to a given radiation dosage and temperature, andthen separating a polymer from each irradiated mixture, wherein themolecular weight of each of the separated polymers is a function of theLewis base concentration in each of said mixtures.

2. A method for controlling the molecular weight of a polymer producedby high-energy radiation of an ethylenically unsaturated monomer whichcomprises:

a. forming from one to a series of mixtures of an ethylenicallyunsaturated monomer having at least one electron attracting groupselected from the group consisting of CN, CONH N0 and COOR where R is analkyl group, attached to the ethylenic linkage of said monomer, and

b. a Lewis acid, soluble in said monomer, at a molecularweight-determining concentration, said acid being selected from thegroup consisting of hydrogen bromide, hydrogen chloride, carbon dioxide,organic alcohols, glycols, amides, protonic solvents, and alkyl or arylderivatives of boron which possess electron-deficient centers,irradiating each mixture with high-energy radiation to a given radiationdosage and temperature, and then separating a polymer from eachirradiated mixture, wherein the molecular weight of the separatedpolymer is a function of the Lewis acid concentration in each of saidmixtures.

t: t I i i

2. A method for controlling the molecular weight of a polymer producedby high-energy radiation of an ethylenically unsaturated monomer whichcomprises: a. forming from one to a series of mixtures of anethylenically unsaturated monomer having at least one electronattracting group selected from the group consisting of -CN, CONH2, NO2,and COOR where R is an alkyl group, attached to the ethylenic linkage ofsaid monomer, and b. a Lewis acid, soluble in said monomer, at amolecular weight-determining concentration, said acid being selectedfrom the group consisting of hydrogen bromide, hydrogen chloride, carbondioxide, organic alcohols, glycols, amides, protonic solvents, and alkylor aryl derivatives of boron which possess electron-deficient centers,irradiating each mixture with high-energy radiation to a given radiationdosage and temperature, and then separating a polymer from eachirradiated mixture, wherein the molecular weight of the separatedpolymer is a function of the Lewis acid concentration in each of saidmixtures.